A study from the University of Copenhagen shows that wearing an immobilizing knee brace for just two weeks caused men in their 20s to lose 22 to 34 percent of their leg muscle strength, while men in their 60s lost 20 to 26 percent (Journal of Rehabilitation Medicine, June 26, 2015).

It took them six weeks of exercising on a bicycle 3-4 times a week to restore the leg muscle size and ability to exercise, but even that did not fully restore the leg muscle strength. This study shows that:
• When muscles of young men are immobilized for two weeks, they lose strength equivalent to aging 40 or 50 years.
• As soon as you stop moving, your muscles start to weaken.
• The larger your muscles, the more muscle you have to lose.
• Younger and stronger people lose strength even faster than older people.
Other studies show that it takes three times the amount of time you were inactive to regain the muscle size that you have lost, probably because you were immobilized 24 hours a day and you exercise for only a short time each day.

Aging Weakens Muscles Even if You Exercise
Inactivity at any age causes muscles to weaken, and you are likely to lose muscle strength with aging no matter how much you exercise. Every muscle in your body is made up of thousands of muscle fibers just as a rope is made up of many strands. Every muscle fiber is innervated by a single nerve fiber. With aging you lose nerves, and when you lose a nerve attached to a muscle fiber, that muscle fiber is lost also. A 20-year-old person may have 800,000 muscle fibers in the vastus lateralis muscle in the front of his upper leg, but by age 60, that muscle would have only about 250,000 fibers. For a 60-year-old to have the same strength as a 20-year-old, the average muscle fiber needs to be three times as strong as the 20-year-old's muscle fibers. You cannot stop this loss of the number of muscle fibers with aging, but you certainly can enlarge each muscle fiber and slow down the loss of strength by exercising muscles against progressive resistance using strength-training machines or by lifting weights (Experimental Gerontology, August 13, 2013).

How to Strengthen Muscles
To enlarge and strengthen muscles, you need to exercise them against resistance forceful enough to damage the muscle fibers. You will know that you have done this because you will feel a burning in the stressed muscle during the later lifts and on the next day, your muscles will feel sore. Then you lift lighter weights, or engage in other sports, for as many days as it takes for your muscles to heal. You can tell this has happened when the soreness goes away.

My Recommendations
We are all likely to have some enforced periods of inactivity, but if you realize how quickly you lose muscle strength and how much longer it takes to gain it back, you will avoid voluntary inactivity as much as possible. For example, when you plan a vacation, make sure it involves physical activity that is at least equal to your regular exercise program.

As you age, expect to become weaker and more likely to fall and break your bones. To enlarge muscles and slow the natural loss of strength with aging, you should lift weights a few times in a row or lighter weight many times in a row. You can become quite strong by using 10 to 15 strength-training machines (for different muscle groups) three times a week. Do two or three sets of 10 repetitions on each machine. Always stop immediately if you feel any pain, tearing or excessive burning. See Making Muscles Stronger.

Caution: Check with your doctor before starting a new exercise program or changing the intensity of your existing program.

About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities.
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